GNU bug report logs - #79990
[PATCH v1] Add binary format specifications '%b' and '%B'

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "Jacob S. Gordon" <jacob.as.gordon <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: [PATCH v1] Add binary format specifications '%b' and '%B'
Date: Thu, 11 Dec 2025 14:42:44 -0500

[Message part 1 (text/plain, inline)]

Tags: patch

Tags: patch

Hey everyone,

For quick things in elisp I often use a small helper function to
format binary digits.  C23 now recognizes ‘%b’ and ‘%B’ for the
‘printf’ functions, and some Lisps have similar directives (e.g. ‘~b’
and ‘~B’ in CL, Guile Scheme), so it would be nice to have this in
‘format’.  Attached is a first draft of a patch achieving this.

[v1-0001-Add-binary-format-specifications-b-and-B.patch (text/patch, attachment)]

[Message part 3 (text/plain, inline)]

Thanks,

-- 
Jacob S. Gordon
jacob.as.gordon <at> gmail.com
Please avoid sending me HTML emails and MS Office documents.
https://useplaintext.email/#etiquette

Message #8 received at 79990 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: "Jacob S. Gordon" <jacob.as.gordon <at> gmail.com>,
 Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 79990 <at> debbugs.gnu.org
Subject: Re: bug#79990: [PATCH v1] Add binary format specifications '%b' and
 '%B'
Date: Fri, 12 Dec 2025 10:04:04 +0200

> From: "Jacob S. Gordon" <jacob.as.gordon <at> gmail.com>
> Date: Thu, 11 Dec 2025 14:42:44 -0500
> 
> For quick things in elisp I often use a small helper function to
> format binary digits.  C23 now recognizes ‘%b’ and ‘%B’ for the
> ‘printf’ functions, and some Lisps have similar directives (e.g. ‘~b’
> and ‘~B’ in CL, Guile Scheme), so it would be nice to have this in
> ‘format’.  Attached is a first draft of a patch achieving this.

Thanks.

If this relies on C23-compliant libc implementation of printf (as I
think it does), then I don't think we want to have a feature in Emacs
which can be used on a small subset of platforms.  We need instead
either to provide our own implementation of that or maybe use Gnulib
printf.

Adding Paul for discussing the possible implementations and the need.

> +@item %b
> +@item %B
> +@cindex integer to binary
> +Replace the specification with the base-two or binary representation of
> +an integer.  Negative integers are formatted in a platform-dependent
> +way.

Don't we want the negative values to be formatted the same on all
platforms?  And if we leave this platform-dependent, then how will
tests you add work on the platforms where the result is different from
what the tests assume?

>       @samp{%b} uses lower case and @samp{%B} uses upper case in the
> +alternate display form prefixes @samp{0b} and @samp{0B}, respectively.
> +The object can also be a floating-point number that is formatted as an
> +integer, dropping any fraction.

Does this support bignums?

Message #11 received at 79990 <at> debbugs.gnu.org (full text, mbox):

From: "Jacob S. Gordon" <jacob.as.gordon <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>, Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 79990 <at> debbugs.gnu.org
Subject: Re: bug#79990: [PATCH v1] Add binary format specifications '%b' and
 '%B'
Date: Fri, 12 Dec 2025 11:23:59 -0500

On 2025-12-12 03:04, Eli Zaretskii wrote:
> If this relies on C23-compliant libc implementation of printf (as I
> think it does), then I don't think we want to have a feature in
> Emacs which can be used on a small subset of platforms.

Oh you’re right, the fixnum path does call out to sprintf.  I agree
this should be withheld unless it can be made sufficiently portable.

>> +an integer.  Negative integers are formatted in a platform-dependent
>> +way.
>
> Don't we want the negative values to be formatted the same on all
> platforms?  And if we leave this platform-dependent, then how will
> tests you add work on the platforms where the result is different from
> what the tests assume?

I took this warning from the %o and %x descriptions, which seems to be
associated with the ‘binary-as-unsigned’ option.  IIUC the default
behaviour is portable, so I think these statements could be sharpened
or combined into a paragraph about ‘binary-as-unsigned’.  Is this
correct Paul?

>>       @samp{%b} uses lower case and @samp{%B} uses upper case in the
>> +alternate display form prefixes @samp{0b} and @samp{0B}, respectively.
>> +The object can also be a floating-point number that is formatted as an
>> +integer, dropping any fraction.
> Does this support bignums?

Yes, the bignum path calls bignum_to_c_string -> gmp_lib::mpz_get_str
with base 2.

   (let ((x (1- (expt 2 128)))) (and (bignump x) (format "%#B" x)))

Best,

--
Jacob S. Gordon
jacob.as.gordon <at> gmail.com
Please avoid sending me HTML emails and MS Office documents.
https://useplaintext.email/#etiquette

Message #14 received at 79990 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Eli Zaretskii <eliz <at> gnu.org>, "Jacob S. Gordon" <jacob.as.gordon <at> gmail.com>
Cc: 79990 <at> debbugs.gnu.org
Subject: Re: bug#79990: [PATCH v1] Add binary format specifications '%b' and
 '%B'
Date: Fri, 12 Dec 2025 10:09:01 -0800

>>  				   + ((p[signedp] == '0'
>>  				       && (p[signedp + 1] == 'x'
>> -					   || p[signedp + 1] == 'X'))
>> +					   || p[signedp + 1] == 'X'
>> +					   || p[signedp + 1] == 'b'
>> +					   || p[signedp + 1] == 'B'))

This looks wrong, as "0b" and "0B" can be produced by hexadecimal 
formats like "%.2X". Better would be to replace the entire expression 
with (!float_conversion && p[signedp] == '0' && p[signedp + 1] == 
conversion), perhaps with a comment explaining that "== conversion".


> We need instead
> either to provide our own implementation of that or maybe use Gnulib
> printf.

Gnulib printf would bring in a lot of stuff: converting floating point 
numbers portably, doing multibyte conversion Gnulib style, etc. Although 
it would work, it's overkill.

Instead, I suggest changing styled_format's 'b' and 'B' processing to 
convert fixnums to bignums and then "goto bignum_arg". On nicer 
platforms where plain printf works (e.g., glibc 2.35+) this step can be 
omitted as an optimization, but that's icing on the cake and could be 
done in a later patch.

Message #17 received at 79990 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: "Jacob S. Gordon" <jacob.as.gordon <at> gmail.com>, Eli Zaretskii <eliz <at> gnu.org>
Cc: 79990 <at> debbugs.gnu.org
Subject: Re: bug#79990: [PATCH v1] Add binary format specifications '%b' and
 '%B'
Date: Fri, 12 Dec 2025 12:14:42 -0800

On 2025-12-12 08:23, Jacob S. Gordon wrote:
> On 2025-12-12 03:04, Eli Zaretskii wrote:
>>> +an integer.  Negative integers are formatted in a platform-dependent
>>> +way.
>>
>> Don't we want the negative values to be formatted the same on all
>> platforms?  And if we leave this platform-dependent, then how will
>> tests you add work on the platforms where the result is different from
>> what the tests assume?
> 
> I took this warning from the %o and %x descriptions, which seems to be
> associated with the ‘binary-as-unsigned’ option.  IIUC the default
> behaviour is portable, so I think these statements could be sharpened
> or combined into a paragraph about ‘binary-as-unsigned’.  Is this
> correct Paul?

Sounds right: %b, %o and %x are all in the same boat.

Come to think of it, how about if we remove binary-as-unsigned instead? 
In practice that experimental variable seems to cause more trouble than 
it hurts. The only response we got[1] to our request for why it's useful 
was actually more an illustration why it's harmful, as the code using it 
mishandled some negative numbers[2].

[1]: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=32252#167
[2]: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=32252#174

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